1. Field of the Invention
The present invention relates to a method and apparatus for determining cement conditions and, more particularly, to such a method and apparatus wherein an acoustic energy source is placed against an interior surface of a casing set within a wellbore to determine the quality of the cement bond, the thickness of the cement, and the acoustic velocity associated with the cement.
2. Setting of the Invention
After a wellbore has been drilled through a subterranean formation, a casing or tubing string is required by statute or regulation to be set therein with a hydraulic bond, as by cementing. Further, it is a requirement that the hydraulic bond be tested to determine if a channel in the cement is present, if a microannulus has formed between the cement and the casing as by shrinkage of the cement, or if the surface of the casing or tubing is defective.
Numerous prior art apparatuses have been developed to determine the quality of the cement bond and are referred to hereinafter as CBL methods and apparatuses. Primarily, the prior art CBL apparatuses include an acoustic transmitting transducer and a plurality of acoustic receivers suspended via a wireline within a wellbore. The acoustic transmitting transducer emits acoustic energy which must pass through the fluid in the wellbore, and into the casing.
In one type of prior art CBL system, acoustic energy encounters the casing and passes therethrough, i.e., the casing acts as a wave guide. The suspended acoustic receivers receive reflected energy and the resulting signals indicate the amount of energy attenuation as the acoustic energy passes through the casing. The better the cement bond with the casing, the greater signal attenuation will result. Conversely, little or no signal attenuation indicates that there are voids in the cement-casing interface.
Another type of prior art CBL system uses pulses of acoustic energy directed at a normal incidence to the casing so that the energy passes through the casing and is reflected back. The suspended acoustic receivers receive reflected energy and the resulting signal amplitudes are analyzed. The better the cement bond with the casing, the lower the signal amplitude. Conversely, a relatively high amplitude indicates a poor cement bond. In both systems, the acoustic receivers convert the acoustic energy reflections into electrical signals, which are preprocessed and transmitted to surface processing equipment to generate a signal amplitude-vs-time plot. A trained well log analyst then can review the plot and determine the quality of the cement bond.
Two of the major problems associated with the prior art methods described above are signal attenuation and transducer misalignment. By placing the acoustic transmitting transducer within the casing but not into contact with the casing, a substantial portion of the transmitted acoustic energy is attenuated by the wellbore fluid and the casing. The fluid's and the casing's impedance are substantially different from that of the transducer and the transmitted energy tends to reverberate within the casing causing it to "ring." Thus, on a plot of the received signals, the important signal representative of the interface between the cement and the casing can be hidden or "smeared" by the resultant signal caused by the casing ringing. Further, the equally critical interface between the cement and the formation can be hidden by the casing ringing or, because so much acoustic energy is lost to the wellbore fluid and casing, this interface is not discernible on the plot at all.
The major problem of transducer misalignment is caused when the pulse-echo transducer is not centered within the casing so that transmitted acoustic energy contacts the casing at a nonperpendicular angle, i.e., at a normal incidence. A substantial portion of the acoustic energy may not penetrate the casing and cement. Further, the energy becomes defocused making image reconstruction difficult.
Another major problem with prior art pulse-echo CBL methods and apparatuses is the frequency needed to penetrate the wellbore fluid is not best for signal interpretation. A low frequency signal is desired to prevent high signal attenuation as it passes out through the wellbore fluid and back, but a low frequency signal is the least desired for signal interpretation. This is because the length of the signal can be greater than that needed to define a particular event, such as the casing-cement interface. The low frequency signal smears the desired amplitude peaks used to define events.
Another major problem with prior art casing attenuation CBL methods and apparatuses is that the energy is transmitted radially and thus received radially. What this means is that one portion of the cement could be bonded adequately to the casing while another portion has no cement at all, which could lead to a catastrophic well failure, but because the signal is radially received, it is an average signal at that depth. The well log analyst would only "see" on the resulting plot an acceptable signal.
A paper entitled "Ultrasonic Cement Bond Evaluation" by R. M. Havira, SPWLA, July 1982 and U.S. Pat. No. 4,255,798 (Havira) both disclose a pulse-echo CBL method wherein a single acoustic transmitting and receiving transducer is suspended within a wellbore. Nowhere is it disclosed or suggested in these disclosures to place an acoustic transmitting and receiving transducer into contact with the interior surface of the casing to eliminate the problems associated with this type of system.
U.S. Pat. No. 3,691,518 (Schuster) and U.S. Pat. No. 3,883,841 (Norel, et al.), both disclose CBL methods wherein an acoustic transmitting transducer is suspended within a wellbore and at least one acoustic receiving transducer is placed into contact with the interior surface of the casing. Nowhere is it disclosed or suggested in Schuster or Norel, et al., to place an acoustic transmitting and receiving transducer into contact with the interior surface of the casing to eliminate the problems associated with this type of system.
U.S. Pat. No. 3,363,719 (Venghiattis) discloses a velocity logging method wherein an acoustic transmitting transducer and an acoustic receiving transducer are placed into contact with the formation material to determine the shear and compressional velocities of the formation. Nowhere is it disclosed or suggested in Venghiattis to place an acoustic transmitting and receiving transducer into contact with the interior surface of a casing or to determine the quality of the cement bond between the casing and the formation.
There is a need for a CBL apparatus and a method of use thereof which can evaluate and determine the cement conditions adjacent a cased wellbore such that a substantial portion of the acoustic energy passes through the casing into the cement and the formation with proper alignment so that the cement conditions can be determined more accurately than previously utilized methods.
The quality of the cement and the condition of the cement bond with the formation are important; however, equally important is the determination of the thickness of the cement and the acoustic velocity associated with the cement. Because the casing is usually not centered within the wellbore, cement may not uniformally surround the casing, i.e., where the casing is closest to the formation, little or no cement may be present. Prior art CBL methods use signal averaging of the cement bond at a certain depth, so the lack of cement at one azimuthal location on the casing may not be detected because its signal would be averaged into the signals from the other locations. Oftentimes cement can include various foreign material picked up during the passage through the wellbore. Therefore, the cement could include irregularities or voids and zones of inadequate strength. As stated above, prior art CBL methods use signal averaging of the cement bond readings, so the variations in cement quality at one azimuthal location may not be detected.
There is a need for a method of determining the thickness of the cement at a particular azimuthal location at a certain depth within the wellbore and a need for a method of determining an indication of the quality of of the cement, such as acoustic velocity, at a particular azimuthal location at a certain depth within the wellbore.